When a customer has an analogue telephony service (so called POTS--Plain Old Telephony Service) and makes a call, the call duration timer starts when the called customer answers. This approach is valid since customers can not get advantage from the call until they are able to converse and this occurs at call answer. The call duration timer stops at release of the call and for normal calls the duration of the call is one of the parameters which affects the amount the calling customer is charged. Calls in an Integrated Services Digital Network (ISDN) domain have followed the approach adopted for POTS, that is the duration timer and hence charging is related to called customer answer. The situation for ISDN calls is not the same as POTS calls since users of the ISDN can misuse some of the ISDN features and gain advantage from the end-to-end signalling and/or bearer connection before the called user answers.
A typical scenario where the ISDN signalling protocol is misused is to perform so-called stealth polling. In this situation, the calling user makes a call of a prearranged type which the called user rejects with a prearranged cause value. The rejection message is transported by the network back to the calling user. The advantage to the calling user is that by receiving the rejection message they know that the called user's equipment and line are working.